1. Field of the Invention
The present invention relates to a process for preparing rolled films or sheets of a crystalline thermoplastic resin in which heat shrinkage is notably improved, and particularly it relates to a process for preparing rolled films or sheets having excellent formability and improved tensile strength in a transverse direction.
2. Description of the Prior Art
Most films and sheets obtained from a crystalline thermoplastic resin by melt extrusion are widely used, for example, directly as various containers, automobile parts, appliance parts, packing materials and the like, and they are subsequently subjected to fabrications such as vacuum forming, pressure forming and press forming.
However, the melt-extruded films or sheets have drawbacks such as low stiffness and poor transparency due to crystallisability.
In order to improve the stiffness and transparency, it has been attempted to subject the films or sheets to rolling treatment. However, when the films or sheets are rolled, crystalline portions and amorphous portions thereof are uniaxially oriented in a longitudinal direction, and as a result, the following problems arise.
The rolled films or sheets have great anisotropy, and therefore when they are subjected to thermal forming such as vacuum forming, pressure forming or press forming, noticeable heat shrinkage occurs in a longitudinal direction, with he result that melt tension increases, which makes the forming operation difficult. In addition, when the preheating temperature is raised so as to lower the melt tension, the sheets break, and thus the forming is impossible. Even if the forming can be made, strain remains in formed articles owing to the strong anisotropy, and for this reason, warp phenomenon and spring back phenomenon take place. Here, the spring back phenomenon is that the formed articles act to return to their shape before formed. As a result of the above-mentioned phenomena, the desirable shape of the formed articles cannot be maintained, and dimensional stability thereof is bad.